As a coating composition to be used in a coil coating of steel plate and the like, various properties including excellent weather resistance, chemical resistance, water resistance, coin scratch resistance, gloss, hardness, and bending properties of the coating, as well as the capability of formulating a high solids paint, are important.
In such application, a polyester resin is a promising material because of resulting in a coating with excellent weather resistance, chemical resistance, hardness and the like. However, this material has a serious drawback of lacking in structural flexibility. As an attempt for giving stretchability and flexibility to a polyester resin, there has been proposed a technique wherein a comparatively long methylene chain is included in either of dicarboxylic acid or diol component or in lactone to be reacted with polyester resin, thereby introducing the same into the polyester's molecular structure. However, hardness and bending properties are, in principle, contrary to each other, and an improvement in flexibility is always linked with a decrease in hardness. Thus, it is quite difficult to attain the well balanced flexibility and hardness in a single polyester resin.
It has also been proposed to use a mixture of soft polyester and hard polyester as a binder resin (Japanese Patent Application Kokai No. 111864/83, ibid 111865/83). In these attempts, while maintaining each of hardness, water resistance and chemical resistance at an appropriate level, success was attained in having an improved flexibility, rubber-like elasticity and the like by the control of methylene chain content of the resinous composition, but it was only accomplished to the satisfaction of the desired properties of the coating on soft materials such as fender, bumper and the like. The proposed coating compositions were of quite dissatisfactory when used as a coil coating of steel plate, especially with respect to hardness, bending properties and the like.
Recently, a very attractive lacquer type coil coating paint composition has been proposed in Japanese Patent Application Kokai No. 210975/84, in which a linear polyester resin having a number average molecular weight of 15,000 to 50,000 and a linear or branched polyester resin having a number average molecular weight of 2,000 to 10,000 are mixed in a weight ratio of 80:20 to 20:80 and used as a resinous binder. This composition is said to give a coating with well balanced hardness and bending processability.
However, since the coating composition does not include any crosslinking agents, the thus formed coating is insufficient in weather resistance, fastness to boiling water, chemical resistance and the like. Even if an attempt is made to blend a crosslinking agent with said coating composition, an additional problem of poor compatibility would inevitably arise from the high molecular weight of the polyester resin used.
Therefore, the application of said lacquer type paint composition is in fact confined in limited areas.
An object of the present invention is therefore to provide a polyester binder which is compatible with a crosslinking agent. An additional object of the invention is to provide a coating composition for coil coating use, which will give a coating with well balanced hardness and bending processabilities, as well as the characteristic properties possessed by polyester resin, and capable of being formulated as a high solids paint.
The inventors, as the results of extensive studies, have found that if a melamine resin at least part of which is methyl methylolated melamine (e. g. methylol melamine methyl ether alone or a mixture of methylol melamine methyl ether and methylol melamine butyl ether) or an isocyanate compound is selected as a crosslinking agent and a combination of low molecular weight (up to number average molecular weight 5000) linear polyester and a branched polyester is used as binder resins, a compatible blend can be formulated and a high solids coating composition capable of resulting in a coating with well balanced hardness and bending processability can be obtained. On the basis of these findings, the present invention has been made.
Thus, according to the invention, there is provided a coating composition for coil coating use consisting essentially of
(a) a binder component comprising a mixture of a linear polyester resin and a branched polyester resin, having a hydroxyl value of 40 to 90, said linear polyester resin being composed of polybasic acid component and polyhydric alcohol component consisting of trihydric and/or polyhydric alcohol (0 to 10 wt% of the solid matter) and dihydric alcohol and having 1.8 to 2.2 hydroxyl groups per molecule and a number average molecular weight of 1000 to 5000, and said branched polyester resin being composed of polybasic acid component and polyhydric alcohol component consisting of trihydric and/or tetrahydric alcohol (3 to 40 weight% of the solid matter) and dihydric alcohol, and having 2.3 to 4.5 hydroxyl groups per molecule, and
(b) a crosslinking component consisting of melamine resin, at least part of which is a methyl methylolated melamine, or isocyanate compound.
The first polyester resin used in the present invention is an oil-free linear polyester resin composed of polybasic acid component and polyhydric alcohol component, the polyhydric alcohol component consisting of substantial amount of dihydric alcohol and a small amount (0 to 10 weight % of the solid matter) of trihydric and/or tetrahydric alcohol, having 1.8 to 2.2 hydroxyl groups per molecule and a number average molecular weight 1000 to 5000.
By the employment of this type of polyester resin, entanglement of resinous molecules and physical crosslinking are of use in giving improved hardness, weather resistance and fastness to boiling water of the coating, and easy unraveling properties of the entangled molecules, sliding properties and stress relaxation are believed to be of use in giving improved bending properties and low temperature impact strength. When the hydroxyl groups are less than 1.8 per molecule, there remains a considerable number of unreacted chains, which will cause a decrease in weather resistance of the coating, whereas if the hydroxyl groups exceed 2.2 per molecule, there will be a decrease in sliding properties and hence the desired bending properties.
The number average molecular weight of said linear polyester resin should be in a range of 1000 to 5000, and preferably 2000 to 4000. This is because, in the combination of said resin with the specified crosslinking agent and branched polyester resin, if the number average molecular weight of said linear polyester is less than 1000, the desired bending processability cannot be expected, and if it exceeds 5000, it is unable to have a high solids coating composition and the desired compatibility with the selected crosslinking agent.
The second polyester resin used in the present invention is a branched, oil-free polyester resin composed of polybasic acid component and polyhydric alcohol component consisting of trihydric and/or tetrahydric alcohol, the hydroxyl groups number per molecule being 2.3 to 4.5. By the selective use of this type of branched polyester resin, hardness, weather resistance and fastness of boiling water of the coating are greatly improved because of the increase in chemical crosslinking between the functional groups contained therein.
If the hydroxyl group number is less than 2.3, it is unable to obtain the desired hardness and weather resistance of the coating, whereas the preparation of branched polyester resin having more than 4.5 hydroxyl groups per molecule is very difficult to do.
In the present invention, the abovementioned linear polyester resin and branched polyester resin are mixed together so as to give a hydroxyl value of 40 to 90. This is because, if the hydroxyl value of the mixed resin is less than 40, the formed coating is too soft to achieve the objects of the invention, and if it exceeds 90, the coating is too hard.
As to the number average molecular weight of said branched polyester resin, it is not so critical to the objects of the invention and is appropriately selected in a common range of about 800 to 4000.
The present branched polyester resin may be modified with a lactone of the formula: ##STR1## wherein n is an integer of 4 to 10, thereby giving softness to the branched polyester resin and increasing crosslinking density of the cured coating, if desired. Said lactone may be used in an amount of 0 to 40 weight% of the resin solid.
Furthermore, in the present invention, either or both of the abovementioned linear and branched polyester resins may be modified with an alkyleneimine such as, for example, N-(2-hydroxy ethyl)ethyleneimine (HEEI) to improve pigment dispersion properties of the resin composition.
As already stated, the most characteristic feature of the invention resides in the point that the abovementioned resin composition is used as a binder component together with a crosslinking agent selected from the group consisting of a melamine resin at least part of which is methyl methylolated melamine, and an isocyanate compound.
Contrary to our expectations, the number average molecular weight of the linear polyester resin to be blended with a branched polyester resin could be markedly lowered when the abovesaid particular compound was selectively used as a crosslinking agent.
At present, the exact reasons have not been fully understood, but the following are presumed to be one of the grounds thereof.
That is, when said crosslinking agent is selected, differing from the case with butylated melamine, the self condensation of the crosslinking agent is prevented and the resin and the crosslinking agent are reacted with each other in alternative order to give a soft, cured coating.
Lowering of the molecular weight of the polyester resin contributes to the improvement in the compatibility of said resin with a crosslinking agent and to the formation of a high solids coating composition. Employment of said polymer blend can result in well balanced properties of hardness and bending processability, and employment of said crosslinking agent by far improves in weather resistance, fastness to boiling water, chemical resistance and marker stain resistance of the coating. Thus, the present composition is very useful as a resinous vehicle of a coil coating paint composition.
As to the blending ratio of the abovementioned polyester resins, it is selected in a wide range depending on the application desired. The polyester blend is preferably compounded with the abovementioned crosslinking agent in a weight ratio (as solid) of 5/95 to 50/50, most preferably 20/80 to 35/65.
The present coating composition is usually applied on a steel plate or the like by coil coating means and baked, in most cases, at 180.degree. to 250.degree. C. for 30 seconds to 2 minutes to give a coating with excellent properties with respect to hardness, bending properties, weather resistance, gloss, coin scratch resistance, fastness to boiling water, chemical resistance and marker stain resistance.